1. Field of the Invention
Aspects of the present invention relate to an image forming apparatus, and, more particularly, to an image forming apparatus including a fixing unit having a belt to improve a fixing performance.
2. Description of the Related Art
An image forming apparatus, such as a printer, a copying machine, a fax machine, and a multifunction apparatus, forms an image on a printing medium according to an input image signal. In an electrophotographic image forming apparatus, which is a type of image forming apparatus, light is scanned onto a photosensitive member charged with a specific potential to form an electrostatic latent image on the surface of the photosensitive member, and the electrostatic latent image is supplied with toner to be developed into a toner image. The developed toner image is transferred to the printing medium, i.e., paper. The toner image is fixed on the paper by heat and pressure while passing through a fixing unit included in the image forming apparatus.
A conventional fixing unit generally includes a heating roller having a heat source therein, and a press roller which is in contact with the heating roller to form a fixing nip at a contact portion between the heating and the press rollers. Accordingly, when the paper with the toner image transferred thereto enters between the heating roller and the press roller, which are rotating, the toner image is fixed to the paper by the heat transferred from the heating roller and the pressure exerted between the heating roller and the press roller.
In the conventional fixing unit, however, since the heating roller has a large thermal capacity, a considerable time is required to heat the heating roller to a fixing temperature resulting in a long warm-up time for the fixing unit. Further, when the width of a fixing nip is expanded to improve the fixing performance, since the press roller and the heating roller have an increased diameter to increase the width of the nip, the apparatus becomes large in size.
Recently, a fixing apparatus using a belt has been proposed. FIG. 1 illustrates a cross-sectional view showing a configuration of a conventional belt-type fixing unit. As shown in FIG. 1, generally, the conventional belt-type fixing unit includes a fixing belt 3, which has an inner surface supported by a first roller 1 and a second roller 2, that circulates and moves about the first and second rollers 1 and 2, and a press roller 4, which is pressed against the first roller 1 and the second roller 2 while the fixing belt 3 is disposed therebetween. A heat source 5 is installed on at least one of the first roller 1 and the second roller 2, which support the fixing belt 3, to thereby heat the fixing belt 3.
According to the above-mentioned belt-type fixing unit, it is possible to ensure a large width of the fixing nip between the fixing belt and the press roller, thereby improving the fixing performance, while maintaining a small size of the fixing unit. Further, since a fixing belt having a relatively low thermal capacity is used as a medium which transfers heat to the paper, it is possible to reduce the warm-up time some extent.
However, in the above-mentioned belt-type fixing unit, since it takes quite a time to increase a whole area of the fixing belt, which circulates and moves between the rollers, to the required fixing temperature, such a fixing unit has a limit in reducing the warm-up time. Further, the fixing belt loses heat to the paper or the surrounding air, and the temperature of the fixing belt rapidly decreases in the circulation process. In this case, since heat is not quickly supplied to the fixing belt, there is a problem of causing a fixing failure due to a decrease in the temperature of the fixing belt. Particularly, the problem is accelerated speed of image forming increases.
To overcome such limits of the belt-type fixing unit, Japanese Patent Laid-open Publication No. 2003-297529 discloses a belt-type fixing unit wherein heat sources are installed inside both the first and second rollers, which support the fixing belt, and an induction heating element or a planar heating element is installed to locally heat a contact portion between the fixing belt and the paper or air. According to the fixing unit disclosed therein, it is possible to quickly heat the fixing belt by the heat sources installed in the two rollers or the heating element installed between the rollers. However, the fixing unit disclosed therein has a problem of large power consumption because a number of heat sources are used to quickly heat the fixing belt and maintain the temperature. Further, it is difficult to manufacture a small-sized fixing unit due to a complicated inner configuration of the fixing unit. Moreover, in the fixing unit disclosed therein, since two rollers are used to support the fixing belt, when the fixing belt is extended due to an increase in the temperature of the fixing belt, there is a large possibility of causing a jam or an image defect due to meandering or corrugation of the belt.